Submicron resolution of secondary radiation in LIGA polymethyl-methacrylate resist exposure

Secondary radiation during LIGA polymethyl-methacrylate (PMMA) resist exposure adversely affects feature definition, sidewall taper, and overall sidewall offset. Additionally, it can degrade the resist adjacent to the substrate, leading to the loss of free-standing features through undercutting during resist development or through mechanical failure of the degraded material. The source of this radiation includes photoelectrons, Auger electrons, fluorescence photons, etc. Sandia's Integrated Tiger Series (ITS), a coupled electron/photon Monte Carlo transport code, is used to compute dose profiles within 1 to 2 μm of the absorber edge and near the interface of the resist with a metallized substrate. The difficulty of submicron resolution requirement was overcome by solving a few local problems, having carefully designed micron-scale geometries. The results for a 10-keV x-ray photons source indicate a 2-μm dose transition region near the absorber edge, resulting from PMMA photoelectrons. This region leads to sidewall offset and to tapered sidewalls following resist development. The results also show a dose boundary layer of around 1 μm near the substrate interface due to electrons emitted from the substrate metallization layer. The maximum dose at the resist bottom under the absorber can be very high and can lead to feature loss during development. This model is also used to investigate resist doses resulting from multilayer substrate.